ch 7: Atoms and Spectra

By analyzing the light received from a star,
astronomers can retrieve information
about the star’s:
– Total energy output
– Surface temperature
– Radius
– Chemical composition
– Velocity relative to Earth
– Rotational period

1. Understanding Atoms

Atom

• Nucleus: Contains protons (positive charge) and neutrons (no charge).

• Electron Cloud: Contains electrons (negative charge) moving around the nucleus.

Kinds of Atoms

• Element: A type of atom defined by the number of protons (e.g., Hydrogen has 1 proton, Carbon has 6).

• Isotope: Same element, but different number of neutrons (e.g., Carbon-12 vs. Carbon-13).

• Ion: An atom that has gained or lost electrons, making it charged.

Common Misconception

• Myth: Matter is solid.

  • Truth: Atoms are mostly empty space!

2. Light and Energy in Atoms

Electrons and Energy Levels

• Electrons move in specific orbits (energy levels).

• Electrons can jump to higher levels by absorbing energy.

• When electrons fall back to lower levels, they emit light (photons).

Interactions of Light and Matter

1. Absorption: An electron absorbs energy and moves up an energy level.

2. Emission: An electron releases energy (as a photon) and moves down an energy level.

3. Ionization: Electron leaves the atom completely if it absorbs enough energy.

3. Types of Spectra

• Spectrum: The range of light wavelengths emitted or absorbed by an object.

Three Types of Spectra (Kirchhoff’s Laws)

1. Continuous Spectrum (all colors): Comes from hot, dense objects (e.g., stars, heated metal).

2. Emission Spectrum (bright lines on dark background): Comes from hot, low-density gases (e.g., neon signs, emission nebulae).

3. Absorption Spectrum (dark lines on colorful background): Happens when light passes through a cooler gas (e.g., the Sun’s spectrum).

Why Are Spectra Important?

• Every element has a unique set of spectral lines.

• Scientists use spectra to identify elements in stars and galaxies.

4. Blackbody Radiation (How Hot Objects Glow)

• Hot objects emit light!

• The color of light depends on temperature.

Key Laws of Blackbody Radiation

1. Stefan-Boltzmann Law (Hotter objects emit more energy)

   • Formula:

   • Hotter => More energy is emitted

2. Wien’s Law (Hotter objects appear bluer)

   • Formula:

   • is the wavelength of peak emission.

   • As temperature increases, the peak shifts towards blue (shorter wavelengths).

Common Misconception

• Myth: Temperature and heat are the same.

  • Truth: Temperature measures particle motion, while heat is energy transfer.

5. The Doppler Effect (Measuring Motion in Space)

• The Doppler Effect tells us if an object is moving toward or away from us.

How It Works

• If an object moves toward us, light waves are compressed → Blue shift.

• If an object moves away from us, light waves are stretched → Red shift.

• The greater the shift, the faster the object is moving.

Formula for Doppler Shift

• = Change in wavelength

• = Original wavelength

• = Velocity of the object

• = Speed of light (3 × 10⁸ m/s)

6. Key Formulas for Quiz & Homework

Final Summary

• Atoms consist of protons, neutrons, and electrons.

• Electrons absorb or emit energy, creating spectral lines.

• Spectra help scientists identify elements and study stars.

• Hotter objects emit more energy and shift toward blue light.

• The Doppler Effect tells us about the motion of objects in space.